For a smoke alarm...well, actually one of those heat alarms that you use in the kitchen that's less sensitive to steam and cooking stuff. Safelincs Ei3103RF
formatting link
, two years old. In my elderly mother's kitchen. Started beeping a couple of weeks ago. Once every 30 seconds or perhaps every minute. I didn't time it. Replaced the battery (9 volt) with new. Seemed OK. Beeping again today. Dammit! Replaced the battery
*again* using the second battery in the twin-pack. Didn't cure the beeping. "But it's a brand new unused battery, right type and everything! I know, I'll take the battery out; that'll shut it up!" Did it heck!. Carried on beeping as before!!
How can that be? Do these things have some sort of storage capacitor in them that keeps supplying a little charge until it too finally goes flat? I'm puzzled!